1. Field of the Invention
The invention relates to a method of creating a particle size distribution model, a method of predicting degradation of a fuel cell catalyst, using the method of creating the particle size distribution model, and a method of controlling a fuel cell, using the method of predicting degradation of the fuel cell catalyst.
2. Description of the Related Art
Fuel cells are operable to convert chemical energy directly into electrical energy, by supplying fuel and an oxidizing agent to two electrodes that are electrically connected, and electrochemically causing oxidation of the fuel. Unlike thermal power generation, fuel cells are free from constraints of the Carnot cycle, and thus exhibit a high energy conversion efficiency. A fuel cell generally consists of a plurality of single cells laminated or stacked together, and each of the single cells has a basic structure in the form of a membrane electrode assembly in which an electrolyte membrane is sandwiched by and between a pair of electrodes. In particular, a polymer electrolyte fuel cell using a solid polymer electrolyte membrane as the electrolyte membrane has the advantages of being easily reduced in size and operating at low temperatures, and is therefore noteworthy for its use as a portable power supply or a power supply for a mobile unit.
In the polymer electrolyte fuel cell, a reaction of the following formula (I) proceeds at the anode (fuel electrode) when hydrogen is used as the fuel.H2→2H++2e−  (I)Electrons generated in the reaction of the above formula (I) pass through an external circuit, do work at an external load, and then reach the cathode (oxidant electrode). Protons generated in the reaction of the above formula (I) transfer by electrical permeation from the anode side to the cathode side in the solid polymer electrolyte membrane while they are in a hydrated state.
Also, a reaction of the following formula (II) proceeds at the cathode when oxygen is used as the oxidizing agent.2H++(½)O2+2e−→H2O  (II)Water formed at the cathode passes mainly though a gas diffusion layer, and is discharged to the outside. Thus, the fuel cell is a clean power generator since nothing but water is discharged from the fuel cell.
FIG. 9 schematically shows a cross-section of a single cell 100 of a general polymer electrolyte fuel cell when it is cut in a direction of lamination of layers. The single cell 100 includes a membrane electrode assembly 8 consisting of a solid polymer electrolyte membrane (which may be simply called an electrolyte membrane) 1 having hydrogen ion conductivity, and a cathode 6 and an anode 7 between which the electrolyte membrane 1 is sandwiched. The single cell 100 further includes separators 9 and 10 located outwardly of the electrodes (i.e., the cathode 6 and anode 7), respectively. The membrane electrode assembly 8 is sandwiched by and between the separators 9 and 10. Gas channels 11 and 12 are formed at the boundaries of the separators and the electrodes, and hydrogen gas is continuously supplied to the anode, while gas (normally, air) containing oxygen is continuously supplied to the cathode. Generally, each electrode consists of a catalyst layer and a gas diffusion layer, which are laminated in this order as viewed from the electrolyte membrane. Namely, the cathode 6 consists of a cathode catalyst layer 2 and a gas diffusion layer 4 that are laminated on each other, and the anode 7 consists of an anode catalyst layer 3 and a gas diffusion layer 5 that are laminated on each other.
One of the problems encountered in the polymer electrolyte fuel cell is voltage reduction caused by dissolution of catalyst metal in the electrodes. With regard to this problem, a mathematical model that simulates oxidation and dissolution of a platinum catalyst when it is used as a catalyst metal is discussed, and calculation results using this model are described in a non-patent document (R. M. Darling and J. P. Meyers: J. Electrochem. Soc., vol. 150, pages A1523-A1527, 2003).
In the above-identified non-patent document, the rates of reactions, i.e., oxidation and dissolution, of the platinum catalyst, are specifically discussed. However, even if the mathematical model described in this document is used, precise simulation results that agree with experimental results are not necessarily obtained, as is apparent from FIG. 1 and FIG. 5 of this document.